Modern automotive internal combustion engines comprise an exhaust pipe that terminates in a catalyser, which has the function of reducing levels of pollutants contained in the exhaust gas; in particular, the catalyser stores either the NOx groups produced during combustion, or the sulfur (in the form of SOx), which is contained in the fuel and is released during combustion. The catalyser has limited storage capacity for NOx groups and sulfur (such storage capacity generally amounts to 3-5 grams) and when said storage capacity is exhausted, the catalyser must be cleaned by means of a regeneration process.
The total mass of NOx groups produced during combustion is much greater than the mass of sulfur released during combustion, and moreover the regeneration process to remove NOx groups (a few seconds of rich combustion) is much shorter than the regeneration process to remove sulfur (at least two minutes of rich combustion combined with an internal temperature in the catalyser which, in relative terms, is very high). For the reasons stated above, the regeneration process to remove NOx groups is normally carried out every 45-75 seconds of engine operation, while the regeneration process to remove sulfur is normally carried out every 6-10 hours of engine operation.
In particular, the actual residual capacity available in the catalyser for storing NOx groups is estimated periodically according to the time elapsed since the preceding regeneration process to remove sulfur and according to the sulfur content of the fuel, and performance of the regeneration process to remove sulfur is scheduled on the basis of said estimate of residual capacity.
Fuel manufacturers guarantee the maximum sulfur content of fuel (for example in Italy said value is currently 150 ppm); however, the actual sulfur content is very often below said maximum value, such that using the maximum value results in an, often very significant, overestimate of sulfur content, so resulting in a greater frequency of regeneration, which entails both increased consumption and greater irregularity in engine operation. Moreover, the maximum sulfur content in fuel varies from country to country, as a result of which an engine calibrated to use a fuel in one country might not operate optimally with fuel from another country.
In order to resolve the problems described above, it has been proposed to use a sensor capable of directly measuring the actual sulfur content of the fuel; however, said sensor is particularly expensive and normally requires frequent calibration to provide accurate measurements.